This seems most likely to be an error in the document: "median checking account balances at account creation are only around $3,000 ... At a 4% net interest mar

This seems most likely to be an error in the document: "median checking account balances at account creation are only around $3,000 ... At a 4% net interest margin, this account would only contribute about $120 in margin during its first year...." 4% of $3,000 is $12. I suspect both $350 and $120 are 10x too big. Alternatively the average checking account is 10x the median which seems unlikely.